It is estimated that more than 100,000 American women are diagnosed with estrogen receptor (ER)–positive, postmenopausal breast cancer each year, accounting for about half of all women with breast cancer in the United States. “ER–positive breast cancer is the single most common breast cancer diagnosis in the country,” says Harold J. Burstein, MD, PhD. “One of the most important treatments for women with postmenopausal breast cancer is anti-estrogen therapy.” Filling in Knowledge Gaps Tamoxifen and aromatase inhibitors (AIs) are treatments that can be used as adjuvant therapy after initial surgery, chemotherapy, and/or radiation in an effort to prevent breast cancer recurrences. In 2010, the American Society of Clinical Oncology (ASCO) issued a guideline update on the use of adjuvant hormone therapy for women with hormone receptor-positive breast cancer, revising previous guidelines from 2004. For the most recent update, ASCO’s Endocrine Therapy for Breast Cancer Update Committee conducted a systematic review of the available medical literature to develop the recommendations. “We reviewed the wealth of research that has emerged in the past several years on anti-estrogen drugs,” explains Dr. Burstein, who co-chaired the Update Committee. “Our missions were to fill in gaps in our understanding of how best to use these newer treatments and to gain better clarity on the trade-offs and side effects of these therapies.” ASCO’s updated guideline reviews recent research on both AIs and tamoxifen. Tamoxifen is a selective ER modulator, which blocks estrogen’s ability to reach the ER and stimulate residual cancer growth. AIs work differently in that they deplete the production of estrogen in postmenopausal women. “Tamoxifen and AIs work by different mechanisms and have...